Fasting

Bukhari :: Book 3 :: Volume 31 :: Hadith 116

Narrated Ibn 'Umar:

The Prophet observed the fast on the 10th of Muharram ('Ashura),
and ordered (Muslims) to fast on that day, but when the fasting of
the month of Ramadan was prescribed, the fasting of the 'Ashura'
was abandoned. 'Abdullah did not use to fast on that day unless it
coincided with his routine fasting by chance.